Beyond calcineurin inhibitors: emerging agents in kidney transplantation

Overexpression of PD-1 on T cells promotes tolerance in cardiac transplantation via an ICOS-dependent mechanism

The programmed death 1/programmed death ligand 1 (PD-1/PD-L1) pathway is a potent inhibitory pathway involved in immune regulation and is a potential therapeutic target in transplantation. In this study, we show that overexpression of PD-1 on T cells (PD-1 Tg) promotes allograft tolerance in a fully MHC-mismatched cardiac transplant model when combined with costimulation blockade with CTLA-4–Ig. PD-1 overexpression on T cells also protected against chronic rejection in a single MHC II–mismatched cardiac transplant model, whereas the overexpression still allowed the generation of an effective immune response against an influenza A virus. Notably, Tregs from PD-1 Tg mice were required for tolerance induction and presented greater ICOS expression than those from WT mice. The survival benefit of PD-1 Tg recipients required ICOS signaling and donor PD-L1 expression. These results indicate that modulation of PD-1 expression, in combination with a costimulation blockade, is a promising therapeutic target to promote transplant tolerance.

Stable and destabilized GFP reporters to monitor calcineurin activity in Saccharomyces cerevisiae

The protein phosphatase calcineurin is activated in response to rising intracellular Ca²⁺ levels and impacts fundamental cellular processes in organisms ranging from yeast to humans. In fungi, calcineurin orchestrates cellular adaptation to diverse environmental challenges and is essential for virulence of pathogenic species. To enable rapid and large-scale assessment of calcineurin activity in living, unperturbed yeast cells, we have generated stable and destabilized GFP transcriptional reporters under the control of a calcineurin-dependent response element (CDRE). Using the reporters, we show that the rapid dynamics of calcineurin activation and deactivation can be followed by flow cytometry and fluorescence microscopy. This system is compatible with live/dead staining that excludes confounding dead cells from the analysis. The reporters provide technology to monitor calcineurin dynamics during stress and ageing and may serve as a drug-screening platform to identify novel antifungal compounds that selectively target calcineurin.

Pharmacologic targeting of regulatory T cells for solid organ transplantation: current and future prospects

The last three decades have witnessed significant advances in the development of immunosuppressive medications used in kidney transplantation leading to a remarkable gain in short-term graft function and outcomes. Despite these major breakthroughs, improvements in long-term outcomes lag behind due to a stalemate between drug-related nephrotoxicity and chronic rejection typically due to donor-specific antibodies. Regulatory T cells (Tregs) have been shown to modulate the alloimmune response and can exert suppressive activity preventing allograft rejection in kidney transplantation. Currently available immunosuppressive agents impact Tregs in the alloimmune milieu with some of these interactions being deleterious to the allograft while others may be beneficial. Variable effects are seen with common antibody induction agents such that basiliximab, an IL-2 receptor blocker, decreases Tregs while lymphocyte depleting agents such as antithymocyte globulin increase Tregs. Calcineurin inhibitors, a mainstay of maintenance immunosuppression since the mid-1980s, seem to suppress Tregs while mammalian targets of rapamycin (less commonly used in maintenance regimens) expand Tregs. The purpose of this review is to provide an overview of Treg biology in transplantation, identify in more detail the interactions between commonly used immunosuppressive agents and Tregs in kidney transplantation and lastly describe future directions in the use of Tregs themselves as therapy for tolerance induction.

Calcineurin Targets Involved in Stress Survival and Fungal Virulence

Calcineurin governs stress survival, sexual differentiation, and virulence of the human fungal pathogen Cryptococcus neoformans. Calcineurin is activated by increased Ca²⁺ levels caused by stress, and transduces signals by dephosphorylating protein substrates. Herein, we identified and characterized calcineurin substrates in C. neoformans by employing phosphoproteomic TiO₂ enrichment and quantitative mass spectrometry. The identified targets include the transactivator Crz1 as well as novel substrates whose functions are linked to P-bodies/stress granules (PBs/SGs) and mRNA translation and decay, such as Pbp1 and Puf4. We show that Crz1 is a bona fide calcineurin substrate, and Crz1 localization and transcriptional activity are controlled by calcineurin. We previously demonstrated that thermal and other stresses trigger calcineurin localization to PBs/SGs. Several calcineurin targets localized to PBs/SGs, including Puf4 and Pbp1, contribute to stress resistance and virulence individually or in conjunction with Crz1. Moreover, Pbp1 is also required for sexual development. Genetic epistasis analysis revealed that Crz1 and the novel targets Lhp1, Puf4, and Pbp1 function in a branched calcineurin pathway that orchestrates stress survival and virulence. These findings support a model whereby calcineurin controls stress and virulence, at the transcriptional level via Crz1, and post-transcriptionally by localizing to PBs/SGs and acting on targets involved in mRNA metabolism. The calcineurin targets identified in this study share little overlap with known calcineurin substrates, with the exception of Crz1. In particular, the mRNA binding proteins and PBs/SGs residents comprise a cohort of novel calcineurin targets that have not been previously linked to calcineurin in mammals or in Saccharomyces cerevisiae. This study suggests either extensive evolutionary rewiring of the calcineurin pathway, or alternatively that these novel calcineurin targets have yet to be characterized as calcineurin targets in other organisms. These findings further highlight C. neoformans as an outstanding model to define calcineurin-responsive virulence networks as targets for antifungal therapy.

Calcineurin is a Ca²⁺/calmodulin-dependent protein phosphatase essential for stress survival, sexual development, and virulence of the human fungal pathogen Cryptococcus neoformans and other major pathogenic fungi of global human health relevance. However, no calcineurin substrates are known in pathogenic fungi. Employing state-of-the-art phosphoproteomic approaches we identified calcineurin substrates, including calcineurin itself and the conserved Crz1 transcriptional activator known to function in calcium signaling and stress survival. Remarkably, our study also identified novel calcineurin targets involved in RNA processing, stability, and translation, which colocalize together with calcineurin in stress granules/P-bodies upon thermal stress. These findings support a model whereby calcineurin functions in a branched pathway, via Crz1 and several of the identified novel targets, that governs transcriptional and posttranscriptional circuits to drive stress survival, sexual development, and fungal virulence. Our study underscores C. neoformans as an experimental model to define basic paradigms of calcineurin signaling in global thermostress responsive virulence networks that can be targeted for fungal therapy.

The neurology of solid organ transplantation

Transplantation is the rescue treatment for end-stage organ failure with more than 110,000 solid organs transplantations performed worldwide annually. Recent advances in transplantation procedures and posttransplantation management have improved long-term survival and quality of life of transplant recipients, shifting the focus from acute perioperative critical care needs toward long-term chronic medical problems. Neurologic complications affect up to 30-60 % of solid organ transplant recipients. Common etiologies include opportunistic infections and toxicities of antirejection medications, and wide spectrum of toxic and metabolic disturbances. Most complications are common to all allograft types, but some are relatively specific for individual allograft types (e.g., central pontine myelinolysis in liver transplant recipients). Close collaboration between neurologists and other transplant team members is essential for effective management. Early recognition of complications and accurate diagnosis leading to timely treatment is essential to reduce the morbidity and improve the overall transplant outcome.

Profile of the Pleximmune blood test for transplant rejection risk prediction

The Pleximmune™ test (Plexision Inc., Pittsburgh, PA, USA) is the first cell-based test approved by the US FDA, which predicts acute cellular rejection in children with liver- or intestine transplantation. The test addresses an unmet need to improve management of immunosuppression, which incurs greater risks of opportunistic infections and Epstein-Barr virus-induced malignancy during childhood. High-dose immunosuppression and recurrent rejection after intestine transplantation also result in a 5-year graft loss rate of up to 50%. Such outcomes seem increasingly unacceptable because children can experience rejection-free survival with reduced immunosuppression. Pleximmune test sensitivity and specificity for predicting acute cellular rejection is 84% and 80% respectively in training set-validation set testing of 214 children. Among existing gold standards, the biopsy detects but cannot predict rejection. Anti-donor antibodies, which presage antibody-mediated injury, reflect late-stage allosensitization as a downstream effect of engagement between recipient and donor cells. Therefore, durable graft and patient outcomes also require accurate management of cellular immune responses in clinical practice.

The calcineurin signaling network evolves via conserved kinase-phosphatase modules that transcend substrate identity

To define a functional network for calcineurin, the conserved Ca(2+)/calmodulin-regulated phosphatase, we systematically identified its substrates in S. cerevisiae using phosphoproteomics and bioinformatics, followed by copurification and dephosphorylation assays. This study establishes new calcineurin functions and reveals mechanisms that shape calcineurin network evolution. Analyses of closely related yeasts show that many proteins were recently recruited to the network by acquiring a calcineurin-recognition motif. Calcineurin substrates in yeast and mammals are distinct due to network rewiring but, surprisingly, are phosphorylated by similar kinases. We postulate that corecognition of conserved substrate features, including phosphorylation and docking motifs, preserves calcineurin-kinase opposition during evolution. One example we document is a composite docking site that confers substrate recognition by both calcineurin and MAPK. We propose that conserved kinase-phosphatase pairs define the architecture of signaling networks and allow other connections between kinases and phosphatases to develop that establish common regulatory motifs in signaling networks.